Originally Posted by westhawk
.........the maximum range cruise profile stipulates a reducing mach number increment for each reducing increment of flight weight as fuel is burned off..........
Not always so
Westhawk, but true most of the time. If we are discussing fuel related mission profiles (e.g. Maximum Range Cruise and Long Range Cruise), at the altitudes that we're flying we are slightly above Mcrit, by about .02 to .04 depending upon the aircraft. For example, if Mcrit was 0.72, we'd find MRC typically around M0.74 or so at our Optimum Altitude. Mcrit, therefore, assumes a high degree of significance in establishing the target MRC/LRC speed. Factors affecting Mcrit are therefore important.
Whilst we tend to think of Mcrit for a particular wing as constant, it does vary over a small range. Mcrit depends primarily on two important factors, i.e.
(1) The actual Mach Number (obviously), and
(2) The angle of Attack (AoA).
At light weights through to Medium to Medium + weights, Mcrit varys very little. At very high weights, AoA must be increased because of the greater weight, and with this, even at the same speed as for lower weights, greater acceleration of air over the wing occurs, leading to an earlier encounter with Mcrit. Using 'standard' Mcrit of 0.72 as an example, at the high weight it would not be surprising to encounter Mcrit as low as 0.70 to 0.71. MRC and LRC will still be higher than this 'reduced' Mcrit, but (and it's an important but), MRC/LRC will be LOWER than it would be at a reduced weight.
In summary, because of the above, it's not uncommon to see scheduled MRC/LRC speed at high weights to actually INCREASE as weight reduces due to burn-off. This increasing speed schedule continues to the point where Mcrit becomes relatively constant, after which MRC/LRC decreases with decreasing weight in the conventional sense. This 'abberation of conventionality' is usually found in the top 10% or so of the cruise weight band for the aircraft.
Regards,
Old Smokey